Electromagnetic position-indicator utilizing torsion band suspension with independent tension and nullpoint adjustments



Jan. 20, 1970 R Es ETAL 3,491,297

ELECTROMAGNETIC POSITION-INDICATOR UTILIZING TORSION BAND SUSPENSION WITH INDEPENDENT TENSION AND NULL-POINT ADJUSTMENTS Filed June 23, 1967 2 Sheets-Sheet 1 INVENTORS RUSSELL R. AMES STANLEY W. BROVARNEY GEORGE T SOWLAKIS BY ATTORNEYS WMEWMW Jan. 20, 1970 R. .AMES ETAL 3,491,297

ELECTROMAGNETIC PO ION-INDICATOR UTILIZIN ORSION BAND SUSPENSION TH INDEPENDE TENS AND NULL- NT ADJUSTM S Filed June 23, 1967 2 Sheets-Sheet 2 INVENTORS RUSSELL R. AMES STANLEY W. BROVARNEY GEORGE T. SOWLAKIS BY m ZW u/# ATTORNEYS United States Patent O ELECTROMAGNETIC POSITION-INDICATOR UTILIZING TORSION BAND SUSPENSION WITH INDEPENDENT TENSION AND NULL- POINT ADJUSTMENTS Russell R. Ames, San Jose, Stanley W. Brovarney, San Carlos, and George T. Sowlakis, Santa Cruz, Calif., assignors to American Standard Inc., a corporation of Delaware Filed June 23, 1967, Ser. No. 648,435 Int. Cl. G01r 1/20 U.S. Cl. 324-150 1 Claim ABSTRACT OF THE DISCLOSURE An electromagnetic position-indicating device in which the rotative assembly has drive coils rotating in a basemounted annular air gap, wherein the rotative assembly is suspended from the base by a pair of aligned torsion bands which are secured in tension to the base coincident with the axis of the air gap, and wherein the rotative assembly is caged in the base so as to prevent such transverse movement of the rotative assembly as would cause the drive coils to hit against the sides of the air gap.

BACKGROUND OF THE INVENTION This invention relates to electromagnetic position-indicating devices of the type as disclosed in Gootherts Patent No. 3,200,333.

In general, such devices comprise a base, a magnetic member mounted on the base and having opposed coil pieces, a crescent-shaped armature mounted on the base and having the ends thereof spaced from the pole pieces to define an annular gap. A support member is pivotally mounted to the base for rotation about the axis of the annular gap, and carries two drive coil members which are circumscribed about the armature and which have leg portions disposed in the gap with positive clearance between the pole pieces and the armature. A scanning member, such as a mirror, is also carried by the support member. As electrical energy is supplied to the drive coils, the drive coils, support member and scanning member rotate about the axis of the air gap to an amount proportional to the amount of energy supplied to the drive coils. A fuller description of the construction and operation may be found in said Patent No. 3,200,333.

The support member of the device shown in said Gootherts patent is mounted to the base by flexure pivot assemblies, each of which consists of a pair of crossed leaf springs, each having an end secured to the base and the other end secured to the rotative support member.

Although such devices as described in Patent No. 3,200,333 have been in successful use in space vehicles for a number of years, the specifications that such devices must meet have become more and more stringent. It has been found that the flexure pivot mounting of the rotative parts has inherent shortcomings that prevent such devices from meeting some of the more stringent specifications presently required.

One of the major drawbacks of the flexure pivot assembly is that there is no easy way to restrain transverse motion of the rotating mirror-drive coil assembly at the pivot points. As a consequence, under severe vibration the rotative assembly will move transversely sufiiciently so that the drive coils in the annular gap will hang against the armature and core pieces and cause premature failure or, at best, inaccuracy of the device.

The transverse motion at the pivot points can be decreased by making the spring stiifer, but, in so doing, the power required to rotate the drive coils will be increased, in turn increasing the weight of the power supply. Addi- 3,491,297 Patented Jan. 20, 1970 tionally, increasing the thickness of the springs will reduce the life expectancy of the springs due to fatigue of the springs.

Flexure pivot assemblies also have a drawback in that they have an adverse effect on the linearity of the device. The best linearity will be obtained if the center of rotation of the drive coils remains precisely aligned with the axis of-the air gap. However, it has been found that the flexure pivot assemblies will cause slight excursions of the axis of rotation from the air gap axis as the rotative assembly is rotated throughout its length of travel,

Further, position-indicating devices as shown in said Gootherts patent, wherein flexure pivot assemblies are used, have a drawback because the type of suspension does not enable the mechanical null position of the scanning member to be adjusted relative to the base. Thus, once the rotative assembly is mounted to the base by the flexure pivot assemblies, the mechanical null point is fixed. In some present applications this lack of adjustability of the mechanical null point is quite disadvantageous.

Another problem which has been experienced with flexure pivot assemblies is that they are subject to mechanical hysteresis that adversely affects the accuracy of the system.

Thus, we have found that the use of the flexure pivot assemblies for the support of the rotative assembly in a system shown in the above-mentioned Gootherts patent has the following limitations: they will permit undesirable transverse excursions of the rotative assembly, they do not accurately maintain the center of rotation of the rotative assembly coaxial with the air gap, they do not permit the mechanical null position of the scanning member to be adjusted, and they have an undesirable mechanical hysteresis.

SUMMARY OF THE INVENTION The present invention has been made to overcome the above-described disadvantages of a position-indicating device having flexure pivot assemblies. This has been achieved by the use of a diiferent type of suspension for the rotative assembly, namely, by the use of what is referred to herein as a torsion band suspension.

Such suspension comprises, in general, aligned coplanar torsion bands, in end-to-end relationship. The two adjacent ends of the bands are rigidly secured to the center of the rotative scanning member and drive coil assembly, and the opposite ends of the torsion bands are secured to the base coincident with the axis of the annular air gap. The torsion bands are held in tension and are positively secured to the base so that the secured ends cannot move transversely relative to nor rotate relative to the base. Thus, the rotative assembly is suspended from the base by the torsion bands alone for rotation about the axis of the air gap. The drive coils, when energized, will rotate in the air gap, thus applying a torque to the torsion bands to oppose such movement.

In order to prevent the rotative assembly from moving transversely to an extent wherein the drive coils can hit against the sides of the annular air gap, as may happen under severe vibration, the rotative assembly is provided with circular ends caged into circular openings in the base adjacent the points wherein the torsion bands are secured to the base. A positive clearance is provided between the ends of the rotative assembly and the base openings, which is of an amount less than the clearance between the drive coils and the sides of the air gap. Since the torsion bands are secured to the base against any transverse movement relative thereto, the rotative assembly will rotate throughout its range of rotation without any contact between the ends of the rotative assembly and the base. However, if the device is subjected to severe transverse vibration, the ends of the rotative assembly will be limited in transverse movement by the base openings to an amount insuflicient to allow the drive coils to hit the sides of the air gap.

Again, since the ends of the torsion bands are secured to the base against transverse movement relative thereto, the rotative assembly is constrained to rotate coincidentally with the axis of the air gap, and the linearity is consequently improved.

Further, since the ends of the torsion bands are secured to the base against transverse movement relative thereto, it is thus possible to secure such ends in holders rotatable relative to the base about the axis of the air gap. Thus, by rotating these holders, the mechanical null point of the rotative assembly may be easily adjusted. Once adjusted, the holders are then clamped against further rotation.

It has also been found that the use of a torsion band suspension inherently avoids the inaccuracies caused by mechanical hysteresis.

Other objects and advantages will become apparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings forming a part of this application, and in which like parts are designated by like reference numerals throughout the same,

FIG. 1 is a plan view of an electromagnetic positionindicating device constructed in accordance with the invention.

FIG. 2. is a longitudinal sectional view of the device, taken on line 2-2 of FIG. 1, but with the drive coil being omitted for purposes of clarity.

FIG. 3 is an end view in elevation of the device, as seen from line 3-3 of FIG. 2.

FIG. 4 is a transverse sectional view of the device, taken on line 44 of FIG. 1.

FIG. 5 is an end view of the device, as seen from line 5-5 of FIG. 2.

FIG. *6 is a detail sectional view of the device, taken on the line 6-6 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings, which illustrate a preferred embodiment of the device, the electromagnetic position-indicating device, referred to generally by the reference numeral 10, comprises a stationary member formed by aluminum base members 11 and 12, permanent magnet 13, opposed laminated pole pieces 14 and 15 and armature 16. The base members 11 and 12 are disposed on either side of the pole pieces and armature, and each base member is provided with upstanding flanges 17 and 18 and an arcuate flange 19 integral therewith. The pole pieces 14 and 15 are bonded, by a suitable bonding agent, to and between the base member flanges 17 and 18, respectively, and the armature 16 is bonded to and between the arcuate base member flanges 19. The pole pieces and armature are thus fixedly supported by the base members in spaced relation from each other. As will be seen in FIG. 4, the opposed pole pieces 14 and 15 and the crescent-shaped armature 16 define an annular air gap 20 between the armature and pole pieces.

Pole piece 14 has a stub projection 21 to support field coil 22, the latter being held in place on the stub projection by keeper bar 23 screwed to the stub projection. Similarly, pole piece 15 carries field coil 24.

The permanent magnet 13 is fastened to the base mem-' bers 11 and 12 by cap screws 26 'and is thereby secured to the pole pieces 14 and 15. A magnetic flux path is thus formed through the permanent magnet 13, the pole pieces 14 and 15, the annular air gap 20 and the armature 16.

Base member 12 is provided with an upstanding post portion 27 integral therewith, having a semi-circular groove 28 in the upper end thereof. A yoke 29 is secured to the upper end of the post portion 27 by cap screws 30, the yoke having a semicircular groove 31 therein. The grooves 28 and 31 form a circular opening in axial alignment with the axis of the annular air gap 20. Similarly, base member 11 has an upstanding post portion 32 and a yoke 33 secured thereto by cap screws 34, the post 32 and yoke 33 forming an opening 35 in axial alignment with the axis of the annular air gap 20.

A Teflon ring 36 is fixedly secured, as by screws 37, to post 27 and yoke 29, and has a circular opening 38 therethrough in alignment with the axis of the annular air gap 20. Similarly. Teflon ring 40 is fixedly secured to post 32 and yoke 33 and has an opening 41 in axial alignment with the air gap. Since the Teflon rings 36 and 40 are fixedly secured to the base members, they may be considered as portions of the base members, insofar as they cooperate therewith to support the rotative assembly of the device.

The rotative assembly comprises an elongated support member having circular end portions 51 and 52 received within the openings through the Teflon rings 36 and 40, respectively. A pair of rectangularly shaped drive coils 53 and 54 circumscribed around the armature 16 are fixed to the support member by brackets 55 and screws 56 so that a leg 33a and 54a, respectively, of each coil is mounted in the annular air gap. An annular clearance is provided between the end portions of the support member and the Teflon rings which is less than the clearance between the drive coils and the sides of the annular air gap. Thus, in normal use, the support member 50 and the drive coils carried thereby may rotate about the axis of the annular air gap without any physical contact between the' end portions of the support member and the Teflon rings, and without any physical contact between the drive coil legs and the side of the annular air gap. In the event of severe transverse vibration, the support member ends may move transversely into contact with the Teflon rings but the transverse movement of the support member will be limited, by the relation of the clearances, so that the drive coil legs cannot engage either side wall of the annular gap.

The support member 50 is provided with a centrally disposed upright stub 57 on which is mounted the tubular member 58 which in turn is fixedly secured to the under surface of the mirror 59.

The rotative assembly is supported onthe stationary assembly by means of a torsion band suspension which comprises a pair of axially aligned torsion bands 60 and 61 which are secured at their adjacent ends to the support member 50 and are secured at their opposite ends to the base member posts 27 and 32.

A plastic member 62, having a non-circular opening 63 fitting onto the complementary shaped portion of support member stub 57, is provided with transverse grooves 64 and 65 on the upper surface thereof. A metal bar 66 is disposed in groove 64 and has an upstanding terminal pin 67 thereon. Torsion band 60 is spot-welded to the under surface of bar 66 and extends therefrom through the groove 68 in the plastic member. Similarly, torsion band 61 is secured to bar 69 and terminal pin 70 and extends from the plastic member without engagement therewith.

Torsion band 60 extends freely through the slotted portion 71 of the support member end 51 and is secured to holder 72. Holder 72 comprises a terminal pin 73 having an annular flange 94 and a flattened shaft portion 75, the torsion band 60 being secured by spot welding to the flattened shaft portion 75. The holder 72 also includes a plastic member having an annular sleeve 76 and a radial flange 77. In assembling the device, the torsion band 60 is first welded to the shaft portion 7 5, the-terminal pin flange 74 is inserted in the plastic sleeve 76 and the torsion band is potted in place, as at 78.

A wave washer 79, of spring material, is disposed between flange 77 and the post 27.

As seen in FIG. 3, the lower portion of flange 77 is provided with an arcuate cut-out 81 into which is received lobe 82 of adjustment member 83, the latter member being secured to post 27 by screw 84. Flange 77 is also provided with spanner wrench grooves 85.

The other torsion band, 61, is similarly secured to terminal pin 87 of holder 88. Holder 88 has a plastic sleeve 89 mounted in the circular opening 35 formed by the post 32 and yoke 33. As best seen in FIGS. 2 and 5, the plastic holder flange 89 is provided with three threaded holes for the reception of adjustment screws 90, and three arcuate slots 91 through which screws 92 extend into the post 32 and yoke 33. Flange 89 is also provided with spanner wrench grooves 93.

By the above-described construction, the opposite ends of the torsion bands 60 and 61 are secured to the stationary assembly in exact axial alignment with the axis of the annular air gap.

The torsion bands are placed and held in tension (desirably about 6 pounds tension) as follows. Holder 88 is moved away from holder 72, as for example by a suitable tool (not illustrated) inserted between flange 89 and post 32, until the desired tension is produced in the torsion bands. Adjustment screws 90 are then turned against post 32 to hold the flange 89 away therefrom. Screws 92 are then tightened to secure the holder 88 against axial or rotational movement relative to the base member 11.

Both holders may be easily adjusted so that the null position of the rotative assembly can be adjusted. Holder 88 can be adjusted by loosening the screws 92 and by rotating the holder with a spanner wrench until the mirror and drive coil assembly is at the desired null point. The screws 92 are then retightened.

Similarly, holder 72 can be rotatively adjusted by loosening screw 84 and by rotating the holder with a spanner wrench until the mirror and drive coil assembly is at a desired null point. The lobe 82 is then fixed by tightening screw 84, so that holder 72 is held against rotation. Holder 72 is permitted a slight amount of axial movement, however, to compensate for the twisting of the torsion bands during rotation of the mirror and drive coil assembly. The wave washer 79 will maintain the desired tension in the torsion springs.

Since both holders 72 and 88 secure the ends of the torsion bands coaxially thereof and since both holders are -rotatively mounted in the base member posts coaxially of the annular air gap, the null adjustment described above does not disturb the coaxial relationship of the supported assembly relative to the axis of the air gap.

As has been described, the torsion bands are electrically insulated from the base members and the support member 50. Thus, the torsion bands are enabled to form a part of the electrical circuit to the drive coils 53 and 54. A lead wire 94 connects from torsion band terminal 67 to drive coil 54, lead wire 95 connects from drive coil 54 to drive coil 53, and lead wire 96 connects from drive coil 53 to torsion band terminal 70. Thus, the drive coils 53 and 54 are connected in series between the torsion band holder terminals 73 and 87. Suitable wiring is then used to connect these stationary terminals into the desired circuitry.

It is desirable to provide limits of axial rotation of the drive coil and mirror assembly. As best seen in FIG. 6, the support member 50 is provided with a depending stub 97 which rotates with the support member between the stop screws 98 and 99 which are fixed to the post 27. The stop screws 98 and 99 are preferably provided With an eccentric shaft so that the limit of rotation of the support member can be accurately fixed to a desired amount.

The torsion bands 60 and 61 are preferably formed from beryllium copper, such as XHMS-190 grade thereof obtainable from Brush Beryllium Corporation. The thickness of the bands must be sufiiciently great so that the spring constant of the suspended system is sufficiently high that the natural frequency of the system is in the order of from 15 to 30 cycles per second. On the other hand, the bands must be sufficiently thin so that excess power is not required to drive the rotative system. We have found that torsion bands having a range of thickness from .0042 to .0082 inch will give optimal results in a device as illustrated. The widths of the bands are related by empirical equations, and the width will increase as the thickness decreases. For optimal results in the disclosed embodiment, the width will range from .030 to .0625 inch, depending upon the thickness of the bands.

Although in theory torsion bands having a circular or square cross section could be used, we have found that torsion bands having a rectangular cross section are the only practical forms thereof. A circular cross-section band is not desirable because of design limitations. Only the diameter can change and thus it is difficult to obtain the best characteristics for power consumption and natural frequency. In addition, it is 'much more difficult to weld the ends of a round member than it is to weld a rectangular member. A square shaped member has the same disadvantages of a round member in that there is only one cross-sectional dimension that may be varied. With a rectangular torsion band, it is a relatively simple task to select the proper thickness and width in order to obtain the desired natural frequency and power requirements for driving purposes.

The torsion bands are placed under tension, as described above, to aid in restraining the support member 50 against transverse movement. Additionally, it has been found that this preloading eliminates mechanical hysteresis. The preload spring 79 also acts to compensate for any changes in length of the torsion bands due to changes in environmental temperature.

It is to be understood that the specific form of the invention herein shown and described is a preferred embodiment of the same and that various changes may be made in the shape, size and arrangement of parts without departing from the spirit of the invention or the scope of the attached claims.

Having thus described our invention, we claim:

1. In an electromagnetic position-indicating device having a base, a magnet member mounted on said base, and having a pair of opposed pole pieces, a crescent-shaped armature mounted on said base and having the ends thereof spaced from said pole pieces to define an annular gap between the armature and pole pieces, a support member rotatable coaxially with the axis of said annular gap, coil means carried by said support member and circumscribed around said armature with a leg portion of said coil means in said annular gap with positive clearance between said leg portion and said pole pieces and said armature and a scanning member carried by said support members, the improvement comprising:

(a) said base having portions located on opposite sides of said armature, said portions having circular openings in axial alignment with the axis of said annular s p;

(b) said support member extending between said base portions and having circular ends extending into said base portion openings, said ends and openings having an annular clearance therebetween less than the clearance between said coil means leg portion and said pole pieces and armature;

(c) a pair of axially aligned torsion bands in end-to-end relation and coaxial with the axis of said annular gap, said bands extending through the ends of said support member with positive clearance therebetween;

((1) means connecting adjacent ends of said torsion bands to the mid-point of said support member in insulated relationship thereto;

(e) means electrically connecting said coil means to and between said ends of said torsion bands;

(f) insulated holders rigidly secured to each of the other ends of said torsion bands;

(g) means mounting said holders to said base portions for independent axial movement and rotational adjustrnent of said holders relative to said base;

(h) spring means biasing one of said holders in an axial direction away from the other of said holders;

(i) means for securing said other holder relative to said base to prevent axial movement of said other holder from an axially adjusted position thereof towards said one holder and to prevent rotational movement of said other holder from a rotationally adjusted position thereof relative to said base;

(j) means for securing said one holder against rotational movement thereof from a rotationally adjusted position thereof relative to said base While allowing said one holder to move axially in said base.

References Cited UNITED STATES PATENTS 2,733,408 1/ 1956 Wakefield 324- 97 X 3,200,333 8/1965 Gootherts 32497 X 3,333,193 7/1967 Thomander 324154 X 10 RUDOLPH V. ROLINEC, Primary Examiner JERALD J. DEVITT, Assistant Examiner US. Cl. X.R. 

